Methods for producing hollow polymer particles having a single hole include a method wherein polymer particles which have been obtained by uniformly dissolving a hydrophilic monomer, a crosslinkable monomer and an oil-soluble solvent together with a polymerization initiator so as to form a monomer solution, emulsifying and dispersing the monomer solution in an aqueous phase, and subsequent polymerization are then heated so as to volatilize the oil-soluble solvent. In this production method, a phase separation effect between the polymer to be formed and the oil-soluble solvent is employed to form a single hole in the particles.
However, in the hollow polymer particles having a single hole which are obtained by such a method, the particle size distribution depends on the method of mechanical manipulation in emulsification and dispersion. Regulating the outside diameter (particle diameter), inside diameter (single hole diameter), and the outside diameter to inside diameter ratio within fixed ranges has been difficult.
Even when a classifying operation is carried out with a sieve or mesh so as to achieve a uniform outside diameter, obtaining hollow polymer particles having a sufficiently uniform outside diameter distribution is difficult. Moreover, even were the outside diameter to be made uniform in this way, achieving a uniform inside diameter would be impossible.
In addition, methods of classifying the resulting hollow polymer particles by hydrodynamic processes involving the use of, for example, differences in specific gravity are known. However, because large outside diameter, large inside diameter (high degree of hollowness) particles and small outside diameter, small inside diameter (low degree of hollowness) particles end up having similar degrees of mobility, the classification of these particles by such a method has not been possible.
To address the above problems, hollow polymer particle production methods which involve having monomer ingredients absorbed by seed particles and inducing polymerization are being investigated. Using such methods, it is believed that hollow polymer particles having a relatively uniform outside diameter can be produced.
For example, Patent Document 1 discloses a method for producing polymer particles having a single inner hole. The production method described in Patent Document 1 includes the step of dispersing polymerizable monomer ingredients that include a crosslinkable monomer, a hydrophilic monomer and other monomers in an aqueous dispersion medium and in the presence of dissimilar polymer particles having a composition differing from that of the copolymer to be obtained using these polymerizable monomer ingredients, having the polymerizable monomer ingredients absorbed by the dissimilar polymer particles, then polymerizing the polymerizable monomer ingredients. In Patent Document 1, illustrative examples of the dissimilar polymer include polystyrenes and copolymers of styrene with at least one selected from among acrylic acid, methacrylic acid, acrylic esters, methacrylic esters and butadiene. Illustrative examples of the crosslinkable monomers include divinylbenzene, ethylene glycol dimethacrylate and trimethylolpropane triacrylate. Illustrative examples of the hydrophilic monomer include acrylic acid, methacrylic acid, methyl methacrylate, 2-hydroxyethyl methacrylate, vinylpyridine, glycidyl acrylate and glycidyl methacrylate. Other exemplary monomers include styrene. However, obtaining hollow polymer particles having an outside diameter and an inside diameter of sufficient uniformity by the method described in Patent Document 1 has been difficult. In the working examples of Patent Document 1 as well, although generally single-hole hollow polymer particles are obtained, neither the outside diameter nor the inside diameter were uniform. Moreover, in addition to particles having a single-hole structure, particles having a plurality of holes were also present.
Patent Document 2 discloses a method for producing hollow polymer particles having single inner holes with an average inner hole diameter from 0.25 to 0.85 time the average particle diameter of the particles by emulsion polymerizing (B) a monomer component which includes (B-a) an ionic monomer, (B-b) a nonionic monomer having a solubility parameter which decreases as the monomer changes to a polymer during polymerization and (B-c) a nonionic monomer other than the foregoing nonionic monomer (B-b). The emulsion polymerization is carried out in an aqueous medium, using a water-soluble polymerization initiator, at a polymerization temperature which satisfies certain conditions, and in the presence of (A) polymer particles obtained by polymerizing a monomer which includes (A-a) an ionic monomer, (A-b) a nonionic monomer having a solubility parameter which either does not change or increases as the monomer changes to a polymer during polymerization and (A-c) a nonionic monomer other than the foregoing nonionic monomer (A-b). Moreover, the production method described in Patent Document 2 is characterized in that, of the combination of above polymer particles (A) and monomer component (B), the absolute value of the difference between the solubility parameter [δ(A-b), p] of the polymer obtained by polymerizing the nonionic monomer serving as component (A-b) and the solubility parameter [δ(B-b), m] of the nonionic monomer serving as component (B-b) is 1.0 or less. However, in the method described in Patent Document 2, it was not possible to obtain hollow polymer particles having both outside diameters and inside diameters of sufficient uniformity. Moreover, in the production method described in Patent Document 2, because a water-soluble polymerization initiator is used, the content of polymerization initiator within the monomer oil droplets is small. As a result, not only does the conversion to the polymer decrease, emulsion polymerization also arises in the aqueous phase, which has resulted in the admixture of particles having no inner hole.
Patent Document 3 discloses a production method in which oil droplets are obtained by injecting a uniform mixture A containing a hydrophilic monomer, a crosslinkable monomer, another monomer and an oily substance through a microporous membrane into a liquid B which is immiscible in A, following which polymerization is carried out so as to obtain particles containing the oily substance as the nucleus. However, although the technique of passing the monomer through a microporous membrane does result in a more uniform outside diameter distribution than methods which use a conventional emulsifying apparatus, there does ultimately exist a need to carry out operations such as classification.